Expanded molded articles made of polystyrene-based resin exhibit excellent cushioning and thermal insulation properties and can easily be molded, and thus they are widely used as packaging materials or thermal insulating materials. However, since their impact resistance and resilience are insufficient, there has been a problem in that they are prone to generating cracks and chips easily and thus s are not suitable for packaging, for example, of precision equipment and the like.
On the other hand, expanded molded articles made of polypropylene-based resin exhibit excellent impact resistance and resilience, but require heavy equipment and facilities when molded. Further, the polypropylene-based resin due to its inherent properties, needs to be transported from a raw material manufacturer to a molder in the form of expanded particles. Thus, there has been a problem in that the bulky shipment of such expanded particles leads to an increase in manufacturing costs thereof.
Recently, a rubber-modified styrene-based resin expanded molded article has been proposed that exhibits improved impact resistance and resilience, as well as can be easily molded compared to expanded articles made of polystyrene-based resin, or in other words an expanded molded article made of high impact polystyrene resin (hereinafter referred to as “HIPS”) in which an elastic component such as butadiene rubber is formulated into polystyrene-based resin, has been proposed (for example, refer to Japanese Unexamined Patent Application, First Publication No. S56-67344 (Patent Document 1), Japanese Patent No. 2841303 (Patent Document 2), Japanese Patent No. 4101379 (Patent Document 3), and Japanese Unexamined Patent Application, First Publication No. H03-182529 (Patent Document 4).
For example, Patent Document 1 discloses expanded particles with improved impact resistance made of a resin in which non-orientable rubber particles are dispersed in polystyrene.
However, since the rubber particles are non-orientable, the rubber particles do not readily deform thereby to break-through the membrane of cells inside the thin cell structure of the expanded article. In particular, with pre-expanded particles having a high expansion ratio, there has been a problem in that the retention of the blowing agent gas is insufficient due to said break-through of the rubber particles.
Patent Document 2 discloses an expanded article of a resin in which HIPS and a hydrogenated styrene-butadiene block copolymer are mechanically mixed.
However, since such an expanded article contains a rubber component mechanically mixed, there has been a problem in that if the dispersion of the mixed rubber component is insufficient, the dispersion of the rubber component in the cell membranes of the expanded article becomes uneven inhomogeneous and such cells tends to be broken and opened through readily. This tendency is often seen particularly when the resin is highly expanded, where the expandability of the highly expanded particles is reduced and gaps may be produced between the particles of the expanded molded article, thus resulting in an unfavorable appearance of the expanded molded article. Further, while the impact resistance of said expanded molded article is improved compared to that of a conventional polystyrene-based expanded molded article, it is still not at a practically sufficient level.